In Japanese Patent Laid-open Nos. Sho 50-151997, Sho 50-158680 and Hei 2-178303, sulfonium salt compounds similar to the ones of the present invention are disclosed, wherein it is written that such sulfonium salt compounds can be used as an initiator to cure cationic polymerizable compounds, such as epoxy compounds, under light and radiations, such as electronic rays and X-ray.
However, the sulfonium salt compounds described in Japanese Patent Laid-open No. Sho 50-151997 is known as a highly photoactive polymerization initiator and is useful for curing transparent compositions, but it is not applicable for photomolding where long wavelength laser is used and photo curing of such compositions that contain high content of pigments, etc. because such sulfonium salt compounds have almost no absorption in a wavelength range longer than 360 nm, which is effective for ultraviolet radiation curing. In order to solve such problems, various studies on a sensitizer had been made, and several compounds, such as phenothiazine, anthracene and perilene, have been found out, however, those compounds were not yet satisfactory in such a purpose. Moreover, those compounds are unable to function as a thermal polymerization initiator so that it is hard to apply them for curing of thick films, and those compounds have other disadvantages, such as their complicated production process, high cost and low solubility in monomers. Whereas, the sulfonium salt compounds described in Japanese Patent No. Hei 2-178303 can work as a thermal polymerization initiator and is therefore capable of curing the thick films, however, their capability as a photo polymerization initiator is very weak and their solubility to monomers are low. Whereas, aliphatic sulfonium salt compounds reported by Endo et. al. in IUPAC MACRO 88 Prepr. 90 (1988) can also work as a thermal polymerization initiator and is therefore capable of curing the thick films, however, their capability as a photo polymerization initiator is very weak.
Therefore, it is an object of the present invention to provide a cationic polymerization initiator which is highly sensitive to heating, light, and irradiation of activation energy rays, such as electron rays and X-ray, particularly sensitive to light of which wavelength being longer than 360 nm, a cationic curable composition which is capable of curing in a short time in various film states of which thickness being over a wide range from very thin to thick, and the cured-product obtained therefrom shows to have excellent physicochemical properties and a curing method.